1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor comprising an under-coating layer for use in digital apparatuses, a process for producing the same, and an image-forming apparatus using the same.
2. Description of the Related Art
In general, a process for electrophotography using a photoreceptor with photoconductivity is one of information recording methods utilizing a photoconductive phenomenon of a photoreceptor. After the surface of the photoreceptor is uniformly charged by corona discharge in a dark place, the charge of an exposed portion is selectively discharged by image exposure to form an electrostatic latent image at a non-exposed portion. After that, colored charged corpuscles (toner) are adhered to the electrostatic latent image to generate an image as a visual picture.
In a sequence of these processes, the followings are required as basic characteristics of the photoreceptor: uniformly chargeable at an appropriate electric potential in a dark place; having a potent charge-holding capacity with little discharge in a dark place; and having high photosensitivity to discharge rapidly by photo-irradiation. In addition, high stability and durability are required such as: easy removability of charge from a surface of a photoreceptor to reduce residual electric potential; high mechanical strength and flexibility; unchangeable electrical characteristics in repeated use, such as electrically charged property, photosensitivity and residual electric potential; and durability against such an environment as heat, light, temperature, humidity and ozone.
In the currently practically used electrophotographic photoreceptor, which is constructed by forming a photoreceptive layer over a conductive support, the electric charges on the surface of a photoreceptor are microscopically lost or reduced to generate a defect of image because a carrier injection is readily caused from the conductive support. In order to prevent it, it is effective to coat defects on the surface of the conductive support, improve electrically charged property of the surface of the conductive support and adhesive property of the photoreceptive layer, and enhance easiness of the application, and therefore an under-coating layer is provided between the conductive support and the photoreceptive layer.
Heretofore, layers comprising a variety of resin materials, metallic particles and metal oxide particles have been examined as the under-coating layer. For example, an under-coating layer containing titanium oxide particles has been examined. The known resin materials used in formation of the under-coating layer of a resin single layer include polyethylene, polypropylene, polystyrene, acrylic resin, vinyl chloride resin, vinyl acetate resin, polyurethane resin, epoxy resin, polyester resin, melamine resin, silicone resin, poly (vinyl butyral) resin, polyamide resin, copolymer resin containing two or more of their repeating units, casein, gelatin, polyvinyl alcohol, and ethylcellulose, and particularly, Japanese Unexamined Patent Publication JP-A 48-47344 (1973) discloses that the polyamide resin is preferred.
The electrophotographic photoreceptor having a single under-coating layer of the polyamide resin, however, shows a tendency to decrease the sensitivity and generate such an image defect as fogging due to large accumulation of the residual electric potential. This tendency is particularly remarkable under circumstances of low temperatures and low humidities. In this connection, JP-A 56-52757 proposes to provide an under-coating layer containing surface-untreated titanium oxide particles in order to prevent an image defect caused by the conductive support and reduce the residual electric potential. In addition, JP-A 4-172362 proposes to provide an under-coating layer containing metal oxide particles of which the surface has been treated with a titanate-type coupling agent in order to improve dispersibility of the titanium oxide particles. U.S. Pat. No. 5,391,448 discloses a photoreceptor comprising an under-coating layer for use in analog apparatuses, in which photoreceptor a relationship between the percentage by weight of a non-conductive needle-like titanium oxide particles content to the under-coating layer and the thickness of the under-coating layer is defined. Furthermore Japanese Unexamined Patent Publication JP-A 59-84257 (1984) discloses a photoreceptor comprising an under-coating layer in which titanium oxide powder and tin oxide powder are dispersed. The proposals disclosed in these Publications are still insufficient in characteristics, and accordingly an electrophotographic photoreceptor having much better characteristics is desired. In the under-coating layers containing metal oxide particles, granular metal oxide particles are used.
In producing the electrophotographic photoreceptors, particularly, the photoreceptive layer may be formed by means of a variety of application, such as a spray method, bar-coating method, roller-coating method, blade method, ring method or dip coating method. In particular, the dip coating method, which comprises immersing a conductive support into a vessel filled with an applying solution and pulling out the support at a certain rate or a gradually changing rate to form a desired layer, is utilized in many cases since it is relatively simple and superior in productivity and cost.
Thus, when such a much employed dip coating method is used in production of the under-coating layer, the resin contained in the liquid coating material for forming the under-coating layer is desired to be hardly soluble in a solvent for the coating solution for forming the photoreceptive layer; in general, a resin soluble in alcohols or water is used. The liquid coating material for forming the under-coating layer may be prepared as an alcohol solution or suspension using such a resin, and applied onto a support by immersion to form an under-coating layer.
The electrophotographic photoreceptors which are provided with an under-coating layer containing the surface-untreated titanium oxide particles or under-coating layer containing the metal oxide particles of which the surface is treated with a titanate-type coupling agent are still insufficient in characteristics. Accordingly, the electrophotographic photoreceptors that are much better in sensitivity and durability to produce a faultless image are desired.